Cottonseed is unique among oilseeds in that distributed throughout the oil and protein bearing kernel are numerous small ovoid sacs, commonly known as pigment glands. These pigment glands contain about 35 to 45 percent by weight of gossypol and gossypol like compounds.
By chemical analysis whole mill run cottonseed, with linters removed, contain up to about 1.5 percent of gossypol. Since the hulls contain little or no gossypol, the gossypol content of dehulled kernels is higher. If the protein content, only, of the cottonseed kernels is considered, its content of gossypol may be as high as 3 percent. This is an important consideration because as the protein content of any cottonseed product is increased by the removal of hulls, oil and other nonprotein constituents, the gossypol content will rise proportionally unless concurrent steps are taken to remove gossypol.
Gossypol is a high reactive material, and under the processing conditions normally used including, but not limited to, moisture, heat, pressure and time, the pigment glands of cottonseed are ruptured, the gossypol is discharged and some or most of it combines with various constituents of the meal. The most usual combination appears to be with lysine, one of the essential amino acids present in cottonseed. When combined with gossypol, this essential amino acid is rendered nutritionally unavailable. Two methods of gossypol analysis are presently in use and these methods permit the determination of gossypol with a high degree of accuracy. One method determines the "free" or uncombined gossypol present. The other method determines the "total" gossypol content. The differnece between the two values is referred to as the "bound" gossypol.
Cottonseed pigment glands normally are mechanically strong and resistant to rupture; however, in the presence of moisture, and particularly moisture in combination with heat and pressure, pigment glands readily rupture and discharge their gossypol content which material is thereby brought into intimate contact with the protein, oil, and other constituents making up the kernel.
Currently, cottonseed is processed by mechanical pressing (screw pressing or hydraulic pressing), by solvent extraction with a commercial grade of n-hexane, or by prepress solvent extraction in which a major part of the oil is first removed by screw-pressing followed by solvent extraction of the resulting press cake with commercial n-hexane. The meal or cake produced by any of these processes is typically adjusted to contain 41 percent protein (nitrogen .times. 6.25) by incorporation of cottonseed hulls that contain little or no protein. Some few commercial cottonseed crushing mills produce a meal with about 50 percent protein. The cake or meals just described (41 to 50 percent protein) are destined for use as animal feed. Processing conditions vary considerably in the different mills and can affect, in a significant manner, sometimes adversely, the quality and nutritive value of the cottonseed meal being produced, especially if use as a feed for nonruminants is intended.
The preparation and processing conditions employed in the aforementioned methods all employ in some degree the addition of moisture to either the kernels or the flaked meats, together with heating or cooking and the application of pressure where screw pressing, prepressing, or hydraulic pressing steps are employed. These conditions of processing are in general ideally suited to rupture the pigment glands, liberate the gossypol contained therein into intimate contact with the other kernel components, and promote the reaction of gossypol with the protein constituents of the kernel.
The presence of cottonseed pigments together with attendant processing conditions often cause the crude cottonseed oil produced conventionally to have a color so dark that the normal alkali refining and bleaching will not yield an oil prime in color. Such colored oils must be sold under a price penalty.
There is a limited commercial production of a cottonseed flour intended for human consumption. This is by careful selection of prime whole cottonseed kernels low in gossypol content from an adjacent production line (producing oil and feel grade meals), by the elimination of as many hulls as possible, by diversion of broken and fine kernel fractions (containing much hull material) back to the adjacent production line, and by careful screw pressing of the selected kernels, followed by fine grinding. By such processing, flour product is produced which is higher in quality than the conventional feed grade meal collaterally produced. The flour product is nevertheless much higher in gossypol content, much lower in protein quality, and is much darker than the cottonseed flour produced by this invention.